[fix] unit tests: call searx.search.initialize in test's setUp

Depending on the order the unit tests are executed, the searx.search module is
initalized or not, issue reported in [1]::

    Traceback (most recent call last):
      File "searxng/tests/unit/test_results.py", line 72, in test_result_merge_by_title
        self.container.extend('stract', [fake_result(engine='stract', title='short title')])
      File "searxng/searx/results.py", line 243, in extend
        histogram_observe(standard_result_count, 'engine', engine_name, 'result', 'count')
      File "searxng/searx/metrics/__init__.py", line 49, in histogram_observe
        histogram_storage.get(*args).observe(duration)
        ^^^^^^^^^^^^^^^^^^^^^
      AttributeError: 'NoneType' object has no attribute 'get'

To ensure that the searx.search module is initialized, the

- searx.engines.load_engines is replace by
- searx.search.initialize

[1] https://github.com/searxng/searxng/pull/3932#discussion_r1822406569

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

tests/unit/test_engine_mariadb_server.py

@@ -2,26 +2,11 @@
 # pylint: disable=missing-module-docstring
 
 from unittest.mock import MagicMock, Mock
-from searx.engines import load_engines, mariadb_server
+from searx.engines import mariadb_server
 from tests import SearxTestCase
 
 
 class MariadbServerTests(SearxTestCase):  # pylint: disable=missing-class-docstring
-    def setUp(self):
-        load_engines(
-            [
-                {
-                    'name': 'mariadb server',
-                    'engine': 'mariadb_server',
-                    'shortcut': 'mdb',
-                    'timeout': 9.0,
-                    'disabled': True,
-                }
-            ]
-        )
-
-    def tearDown(self):
-        load_engines([])
 
     def test_init_no_query_str_raises(self):
         self.assertRaises(ValueError, lambda: mariadb_server.init({}))